This invention is broadly concerned with a process for recovery of sulfur from SO.sub.2 -containing gas by the concurrent use of a SO.sub.2 reduction reactor and a Claus reactor, in particular a process for recovery of sulfur wherein the H.sub.2 S/SO.sub.2 molar ratio or (H.sub.2 S+COS)/SO.sub.2 molar ratio of the gaseous mixture introduced into the Claus reactor is maintained at a level of about 2 by controlling the amount of oxygen supplied to the SO.sub.2 -containing gas introduced into the SO.sub.2 reduction reactor in accordance with the SO.sub.2, H.sub.2 S and COS concentrations of the effluent gaseous mixture from the Claus reactor.
Claus process is known from old times as a process for production of an elemental sulfur. This Claus process comprises supplying H.sub.2 S and SO.sub.2 to a Claus reactor packed with a catalyst such as alumina to thereby bring about a reaction: 2H.sub.2 S +SO.sub.2 .fwdarw.3S+H.sub.2 O and recovering an elemental sulfur from the gas resulting from this reaction. In the case of this process, the most essential requisite for yielding sulfur at a maximum efficiency is to maintain the H.sub.2 S/SO.sub.2 molar ratio in the inlet gas of Claus reactor at a level of 2.
Accordingly, when recovering sulfur from exhaust gas using Claus reaction in petroleum refining industry, considering that said exhaust gas contains H.sub.2 S there is adopted an elaborate means for controlling the H.sub.2 S/SO.sub.2 molar ratio in the gas supplied to the Claus reactor to be at a level of 2, for instance, of arranging a conversion furnace on the upstream side of the Claus reactor and converting a part of H.sub.2 S contained in the exhaust gas into SO.sub.2 by the oxidation reaction of oxygen (air) (H.sub.2 S+3/2O.sub.2 .fwdarw.SO.sub.2 +H.sub.2 O) within said conversion furnace.
As the process for recovering sulfur from SO.sub.2 -containing gas coming from the activated carbon regenerator or the like of the dry exhaust gas desulfurization plants there has hitherto been employed a process comprising introducing said SO.sub.2 -containing gas into the SO.sub.2 reduction reactor using carbonaceous particles such as coal, coke or the like as a reducing agent to thereby reduce SO.sub.2 into a vaporous sulfur and separating said sulfur by condensation. However, this process involves problems to be solved such that the sulfur recovery efficiency is not necessarily high.
U.S. patent application Ser. No. 172,590, filed July 28, 1980, now abandoned by Teruo Watanabe and Kazuhiko Yamamoto and assigned to the assignee of the present invention discloses a process for recovering sulfur from SO.sub.2 -containing gas by the arrangement of a Claus reactor on the downstream side of a SO.sub.2 reduction reactor and by joint use of SO.sub.2 reduction and Claus reactions. To sum up, this process for recovering sulfur comprises the steps of first introducing SO.sub.2 -containing gas into a SO.sub.2 reduction reactor for reducing part of SO.sub.2 into H.sub.2 S, a vaporous sulfur and COS, condensing the effluent gas from said SO.sub.2 reduction reactor to thereby separate sulfur, thereafter mixing the residual gas with additional SO.sub.2 -containing gas and introducing the resulting mixture into a Claus reactor, newly producing a vaporous sulfur within said Claus reactor, and thereafter condensing the effluent gas from the Claus reactor to thereby separate sulfur therefrom. The important factor in carrying out this process is to maintain the H.sub.2 S/SO.sub.2 molar ratio or (H.sub.2 S+COS)/SO.sub.2 molar ratio of the gas mixture introduced into the Claus reactor (which will be referred to as Claus ratio hereinafter) at a level of about 2. For that purpose, there is necessity of controlling the temperature distribution within said SO.sub.2 reduction reactor properly so that the composition of effluent gas from the SO.sub.2 reduction reactor may not vary widely. However, the fact is that this temperature control is not necessarily easy technically. In addition thereto, it is usual that the residual gas after removal of sulfur by condensing the effluent gas from the SO.sub.2 reduction reactor is mixed with a part of SO.sub.2 -containing gas by-passed from said gas to be introduced into the SO.sub.2 reduction reactor in order to maintain the Claus ratio of two at the inlet of the Claus reactor. Since this SO.sub.2 -containing gas generally contains dust, it becomes difficult to ensure maintenance and reliability of instruments for use in controlling the flow rate of said bypassed gas.